The invention relates generally to the field of respiratory assist devices such as ventilators. In particular, the invention relates to a ventilator control system and method for controlling a ventilator pneumatic system.
A medical ventilator delivers gas to a patient""s respiratory tract and is often required when the patient is unable to maintain adequate ventilation. Mechanical ventilation is the single most important therapeutic modality in the care of critically ill patients. Known ventilators typically include a pneumatic system that delivers and extracts gas pressure, flow and volume characteristics to the patient and a control system (typically consisting of knobs, dials and switches) that provides the interface to the treating clinician. Optimal support of the patient""s breathing requires adjustment by the clinician of the pressure, flow, and volume of the delivered gas as the condition of the patient changes. Such adjustments, although highly desirable, are difficult to implement with known ventilators because the control system demands continuous attention and interaction from the clinician.
Further, patients requiring ventilatory assistance must overcome airway resistance in the breathing circuit during exhalation. This resistance, combined with the stiffness of the lungs and the thoracic cage under certain pathological conditions, imposes a significant workload upon a patient whose reserves may already be compromised by underlying disease processes. The present invention reduces patient work of breathing without compromising patient ventilation requirements.
The invention relates to a ventilatory assist device that decreases the resistance to exhalation in the exhalation circuit of a medical ventilator. The device adjusts the resistance within the exhalation circuit by generating a negative pressure around a gas exchange reservoir. The negative pressure is then transmitted to the exhalation circuit. In order to keep the resistance constant with varying amounts of exhalation flow, the device varies the amount of applied negative pressure proportionately with increases in exhalation flow.
In one embodiment of the invention, the clinician enters a desired set of values relating to airway pressure, airway resistance or applied negative pressure through a control panel. A microprocessor within the data processing unit of the device compares these values with data for airway pressure, airway resistance and applied negative pressure that have been measured or calculated by sensors within the device. The microprocessor then adjusts the amount of negative pressure to be created within the gas exchange reservoir that communicates with the patient airway. In one embodiment of the device, negative pressure around the gas exchange reservoir is produced within a rigid canister by varying the airflow through a Venturi valve. In this embodiment, a gas flow controller regulates the flow through the Venturi valve in response to signals it receives from the microprocessor within the data processing unit that calculates the amount by which the applied negative pressure is to be changed. A pressure sensor in communication with the ventilatory unit measures the negative pressure applied to the gas exchange reservoir and transmits these data to the data processing unit.
A method of exhalation assist compensates for resistance to gas flow encountered by a patient requiring assisted or controlled ventilation. The method accomplishes this by first determining the instantaneous flow of exhaled gas, the instantaneous pressure within the exhalation circuit and the instantaneous resistance to air outflow so that these data can be compared with desired values entered by a clinician. Negative pressure is applied to the exhalation circuit so as to alter the measured values to reach the desired values.
The term xe2x80x9cventilator control setting structurexe2x80x9d is defined as a collection of information sufficient to control one parameter of ventilation including one or more of: high alarm level, high alarm active, control level, control level active, low alarm level, low alarm active, range level, range level active, and a range target control structure. The range target control structure defines how and why the parameter is to be adjusted automatically within the specified range. The term xe2x80x9ccycle control structurexe2x80x9d is defined as a collection of waveform samples and a ventilator control setting Structure for each parameter. The term xe2x80x9cphase control structurexe2x80x9d is defined as a collection of phase switching rules that defines how the ventilator control settings are to be utilized and a ventilator control setting for each controllable parameter that exists in the ventilator. Each phase has one or more triggers that are tested every cycle (4 Msecs per cycle) to decide which ventilator control setting to use.
The term xe2x80x9cbreath control structurexe2x80x9d is defined as a collection of phase switching rules that defines how and when one ventilatory breath phase is to switch to another ventilatory breath phase and a phase control structure for each phase of breath defined by the specified breath. Breath phases break up a ventilatory breath into as many phases as desired in order to control inspiration, pause, expiration assist and PEEP with any desired level of control for the specified breath. Each breath has one or more triggers that are tested every cycle (4 Msecs per cycle) to decide whether or not to jump to the beginning of a new phase control structure.
The term xe2x80x9cmode control structurexe2x80x9d is defined as a collection of breath switching rules that defines how and when one ventilatory breath is to switch to another ventilatory breath and a breath control structure for each type of breath defined by the specified mode of ventilation. Each mode has one or more triggers that are tested every cycle (4 Msecs per cycle) to decide whether or not to jump to the beginning of a new breath control structure.
The term xe2x80x9ctherapy control structurexe2x80x9d is defined as a collection of mode switching rules that defines how and when one mode of ventilation is to switch to another mode of ventilation and one mode control structure for each ventilation mode defined by the specified therapy. Also, the term xe2x80x9cbreath parameterxe2x80x9d is defined as at least one of a control setting and an alarm setting.